Multi-user multiple input multiple output (MU-MIMO) is employed in IEEE 802.11ac, which is a wireless LAN standard, in order to efficiently perform communication in a downlink direction from the AP to the STA. In the MU-MIMO, one AP can transmit wireless frames simultaneously to a plurality of STAs. Additionally, in order to efficiently perform simultaneous transmission, a technology is also proposed that flow pairing which is effective when compared with characteristics of traffic is performed.
In Non-Patent Document 1, MU-MIMO in a downlink direction or the like, and a procedure for performing simultaneous transmission or wireless frames that are exchanged between the AP and STA when performing the simultaneous transmission are stipulated. Additionally, a procedure is also stipulated in which combinations of STAs that are destinations of the wireless frames which are transmitted with MU-MIMO are grouped, a group ID (GID) is added to each of the resulting groups of combinations, and then the resulting groups of combinations are simultaneously transmitted. In contrast, in Non-Patent Document 2, a group configuration method for performing downlink MU-MIMO that is efficient when compared with the characteristics of traffic in the downlink direction that can be collected in the AP is disclosed.
On the other hand, in order to perform efficient uplink communication from the STA to the AP, simultaneous transmission in an uplink direction by a plurality of STAs is under study. It is considered that multiple access using a space, a frequency, or domains of both makes it possible to realize the simultaneous transmission of the wireless frames from the plurality of STAs to one AP, but as disclosed in Non-Patent Document 3, this technique is not yet established in a wireless LAN system. However, in Non-Patent Document 4, a procedure is proposed in which the AP transmits wireless frame including information, such as a resource scheduling, immediately before the plurality of STAs start the simultaneous transmission and causes the wireless frame to function as a trigger for the simultaneous transmission.    Non-Patent Document 1: IEEE802.11ac, “Wireless LAN Medium Access Control (MAC) and Physical (PHY) Layer Specifications”, December 2013    Non-Patent Document 2: Y. Inoue, S. Shinohara, M. Mizoguchi, and M. Morikura, “Flow-Based User Pairing Scheme for Multi-User Transmissions over WLANs,” Proceedings of the 11th IEEE Vehicular Technology Society Asia Pacific Wireless Communications Symposium (APWCS), Ping Tung, Taiwan, Aug. 28-29, 2014    Non-Patent Document 3: O. Aboul-Magd, “802.11 HEW SG Proposed PAR”, doc.: IEEE802.11-14/0165r1, March 2014    Non-Patent Document 4: Jinyoung Chun, Wookbong Lee, “Uplink multi-user transmission”, 11-13/1388r0, LG Electronics